Auto merge of #122305 - Nilstrieb:target-tiers, r=davidtwco

Add metadata to targets

follow up to #121905 and #122157

This adds four pieces of metadata to every target:
- description
- tier
- host tools
- std

This information is currently scattered across target docs and both
- not machine readable, making validation harder
- sometimes subtly encoding by the table it's in, causing mistakes and making it harder to review changes to the properties

By putting it in the compiler, we improve this. Later, we will use this canonical information to generate target documentation from it.

I used find-replace for all the `description: None`.

One thing I'm not sure about is the behavior for the JSON. It doesn't really make sense that custom targets supply this information, especially the tier. But for the roundtrip tests, we do need to print and parse it. Maybe emit a warning when a custom target provides the metadata key? Either way, I don't think that's important right now, this PR should get merged ASAP or it will conflict all over the place.

r? davidtwco

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